Monoclonal antibodies have been generated that are reactive with the major envelope and internal structural proteins of mouse mammary tumor viruses (MTVs). Different monoclonals were shown to be reactive with type-specific, group-specific, and interspecies determinants of MTVs. The monoclonals to type-specific determinants were used to demonstrate that differences exist between each of six MTVs derived from different strains of laboratory mice (M. musculus). MTVs grown in murine as well as feline cells were tested in order to rule out the possibility that the differences observed were due to host determinants. Antibodies to MTV interspecies determinants were obtained by immunizing mice with type-B retroviruses obtained from M. cervicolor (MC-MTV) and M. cookii (MCo-MTV), and testing subsequent monoclonals for reactivity with MTVs of M. musculus. The monoclonals generated have been used to demonstrate the diversity of expression of individual antigenic determinants among mammary tumors, and within a given tumor mass in primary mammary tumors of various Mus strains and species.